Climate change: Scientific background and process

Climate change

Scientific background and process

Report 2000:1

ISSN: 0804-4562

Knut H. Alfsen

Jan Fuglestvedt

Hans Martin Seip

Tora Skodvin

CICERO Report 2000:1

Climate change

Scientific background and process

Knut H. Alfsen, Jan Fuglestvedt, Hans Martin Seip and Tora Skodvin 21 December, 1999

CICERO

Center for International Climate and Environmental Research

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Abstract

The paper gives a brief description of natural and man-made forces behind climate change and outlines climate variations in the past together with a brief synopsis likely future impacts of anthropogenic emissions of greenhouse gases. The paper also gives a briefing on the background, organisation and functioning of the Inter-govern-mental Panel on Climate Change (IPCC).

Contents

1 INTRODUCTION... 6

2 ON CLIMATE CHANGE ... 7

2.1 Basics... 7

2.1.1 Solar radiation and the carbon-silicate cycle ... 7

2.1.2 Variations in the Earth’s orbit ... 11

2.1.3 The shape and positions of continents... 14

2.1.4 Albedo ... 14

2.1.5 Atmospheric composition ... 14

2.2 Remembrance of things past... 18

2.3 The present ... 22

2.4 Future climate change ... 25

2.5 The effects of the Kyoto Protocol... 26

2.6 Unresolved scientific questions... 28

3 IPCC: THE BACKGROUND, ORGANISATION AND PROCEDURES... 31

3.1 Background: The history of the establishment of IPCC... 31

3.2 The function and products of the IPCC... 32

3.3 The Assessment Process... 34

3.4 Decision Rules and Recruitment Procedures... 37

4 CONCLUDING REMARKS: ON THE NATURE OF THE CLIMATE PROBLEM ... 39

5 REFERENCES... 40

1 Introduction

In its Second Assessment Report (SAR) from 1995 the Intergovernmental Panel on Climate Change (IPCC) concluded that

Although carefully worded, the statement has created a rather heated debate, also among climate scientist. Some well-known and respected scientists disagree that we at this moment in time are able to discern a human influence on the global climate (Pearce, 1997). Also in connection with the production of the last Assessment report, some procedural errors were introduced in the final editing of the summaries of the report. These errors were used extensively by interest groups opposed to climate change policy to discredit the whole report and the organisation producing the report (IPCC).

On this background it is perhaps understandable that many may have come to see IPCC and its reports as mainly political manifestations, to be discussed within the political arena on par with other political topics. Thus, a basic misconception of IPCC has to some degree been spreading, and this in turn has fed scepticism to the whole issue of climate change. Coupled with the notion that climate change is synonymous with global warming, and that some warming may seem desirable at least at Norwegian latitudes, this has resulted in an attitude in certain (mainly political) quarters that climate change is just one more doomsday prophesy from environmental groups.

In this paper we will give some background on the climate change problem by discussing some well understood and some less well understood mechanisms behind changes in the global climate. The likely human impact on climate change is further compared to natural climate change in the past, and we close this first part of the paper with a review of the main outstanding scientific problems in this field. This will hopefully clarify the nature of the problem of climate change. Then, in the second part of the paper, we will try to convey what IPCC is and what it is not and the role of IPCC in the debate on climate change, before we conclude with some comments on the nature of the problem of climate change.

«The balance of evidence, from changes in global mean surface air temperature and from changes in geographical, seasonal and vertical patterns of atmospheric

temperature, suggests a discernible human influence on global climate. There are uncertainties in key factors, including the magnitude and patterns of long-term natural variability.»

2 On climate change

2.1 Basics

The system governing the climate on Earth consists of many sub-systems coupled together in a non-linear fashion. As such, the system is able to distort and amplify external signals affecting the various sub-systems. This is the basic reason why the climate is such a complex issue to study. Although we may have a good deal of knowledge about individual sub-systems, the many interlinkages with other systems make it very hard to predict the overall behaviour of the global climate in response to for instance emissions of the so called greenhouse gases.

An important sub-system is of course the atmosphere itself. However, the state of the atmosphere (temperature, humidity, clouds, distribution of high and low pressure areas, etc.) is affected by and influences the state of other sub-systems such as the oceans, the cryosphere (snow and ice), the biosphere and even the lithosphere (soil, rock, etc.).

Despite these interactions it can be useful to list the main causes of global climate change as follows:

• Variations in solar radiation (the solar ‘constant’)

• Variations in the Earth’s orbit

• The shape and position of the continents

• Volcanic activity

• Variations in the reflections from the Earth’s surface and atmosphere (albedo)

• Changes in the composition of the Earth’s atmosphere due to weathering, volcanic activity and human activities:

- gases - aerosols - cloud cover

These driving forces operate on a number of time scales, from the very long geological time scale to a more ‘human’ and politically relevant much shorter time scale. Below, we will briefly comment on most of these driving forces.

2.1.1 Solar radiation and the carbon-silicate cycle

The solar radiation varies both on short and long time scales. In broad terms the solar intensity has, as a consequence of the natural evolution of a star the size of our sun, increased by approximately 30 percent since the creation of the Earth some 4.6 billion years ago.

The ‘virgin’ atmosphere of the Earth contained much more CO₂ (and probably CH₄) than the present atmosphere. Extreme volcanic activity was an important source of CO₂ in these early ages. The greenhouse effect of these gases helped to keep the Earth warm, although the solar radiation was considerably less intense than today.

The carbon in the atmosphere also helped in stabilising the climate through what is known as the carbon-silicate cycle (see Box 1). When rock erodes, silicate binds to carbon dioxide in the atmosphere and is then transported to the seas where the carbon in the form of calcium carbonate settles in solid form at the bottom of the oceans. Over

increased precipitation in turn leads to more erosion and consequently a stronger sink for atmospheric CO₂. Oppositely, a low CO₂ concentration leads to a colder climate with possible glaciation as a result, which protects the rock from erosion and weaken this sink for atmospheric CO₂. These feedbacks thus tend to stabilise the global

temperature (and also the CO₂ concentration in the atmosphere) in the long run. Other feedback mechanisms work in the opposite direction (see sections below).

Box 1. The carbon-silicate cycle

On the multimillion year scale the carbon cycle is dominated by the following processes:

• uptake of atmospheric CO₂ by the chemical weathering of calcium and magnesium silicate minerals

• weathering of ancient sedimentary organic matter on the continents and the burial of new organic material in marine sediments

• thermal breakdown of carbonate minerals and organic matter via metamorphism, diagnesis and magmatism with transfer of CO₂ back to the earth surface.

The processes may be represented by the following reactions:

CO₂ + CaSiO₃ ↔ CaCO₃ + SiO₂ (1)

CH₂O + O₂ ↔ CO₂ + H₂O (2)

The first equation, read from left to right, represents weathering of silicates. (Ca may, completely or partly, be replaced by Mg.) We note that CO₂ is consumed. Read from right to left the equation represents breakdown of carbonate.

Equation (2) from right to left represents burial of organic material (CH₂O); this may also be denoted net photosynthesis. Read from left to right, it may be denoted

“georespiration”, i. e. oxidation of organic matter after deep burial or uplift of previously sedimented organic matter.

A better understanding of the role of silicates and carbonates in affecting the atmospheric CO₂ levels, is obtained if we look more closely at weathering and breakdown reactions:

Weathering of carbonates may be written as

CO₂ + H₂O + CaCO₃ → Ca²⁺ + 2HCO₃^– (3) Similarly for silicates

2CO₂ + H₂O + CaSiO₃ → Ca²⁺ + 2HCO₃^– + SiO₂ (4)

The bicarbonate will react with calcium to form solid CaCO₃

2HCO₃^– + Ca²⁺ → CaCO₃ + CO₂ + H₂O (5)

Note that (5) is (3) read from right to left. This implies that carbonate weathering does not affect the atmospheric CO₂ concentration in a long-term perspective. Combing (4)

and (5) gives the result of silicate weathering and carbonate formation, i.e. equation (1) (from left to right).

The carbon-silicia cycle is at the heart of a model of long term development in CO₂ concentration developed by Berner and co-workers (see Berner, 1998, for an

overview). They stress that on a scale of some million years, uptake and release of CO₂ must be in close balance. Otherwise the CO₂ concentration will rise to very high values or decrease to values so low that life cannot exist. Results of the developed model are depicted in figure 1.

We can conclude that on a geological time scale, CO₂ concentrations have been far larger than present values. The results are in reasonable

agreement with estimates of CO₂ levels by other methods.

The rapid decline in the CO₂ concentration between 380 and 350 million years before present (myBP) is especially interesting.

Berner argues that during that period vascular plants spread to upland areas. This would greatly increase the weathering rate; he suggests by a factor of 5 – 10.

Since weathering of silicate rocks represents a sink for atmospheric CO₂, there must be a feedback mechanism

preventing the concentration from becoming too low. Accor- ding to Berner, the main feedback is that as the CO₂ concentration decreases, the temperature goes down and the weathering decreases.

An important conclusion from the work by Berner et al. is that the atmospheric CO₂ level has been a key element in the complex of factors that has determined the climate on the earth through millions of years. Decreases in CO₂ levels have counteracted increased weathering due to the evolution of vascular plants and compensated for increasing solar radiation.

There are also more rapid cycles involving the transfer of carbon between reservoirs, see figure 2. The shortest-term cycle is mainly related to the photosynthesis and

Figure 1. Model results of the ratio of CO₂ in the atmos- phere at a given time divided by that at present (RCO-2).

The line with filled circles represents the most likely values; the other lines indicate crude error limits. The rapid decrease between ca 380 and 350 million years before present (myBP) is largely due to the spread of vascular plants. The increase starting around 250 myBP is largely due to mountain uplift. Note that the sun’s radiation has increased substantially during the period. If this is not taken into account, the modelled RCO-2 values would have been lower; in the period before about 400myBP less than half the values shown in the figure. From Berner 1994, fig. 19.

Also on a relatively short time scale, the solar output varies over the so-called solar sunspot cycle lasting for approximately 11 years. Although the total intensity of the solar radiation does not vary much with solar activity measured by number of sun spots or length of the sun spot cycle, it is still possible to observe a good correlation between the length of the sun spot cycle and mean temperature over land in the northern

hemisphere (Friis-Christensen and Lassen, 1991). The correlation deteriorates, however, if the period is extended beyond the mid 1990s. We also lack a convincing explanation of how such small variations in solar output can cause observable effects on the global temperature.

Figure 2. The biogeochemical carbon cycle prior to human inference. One may distinguish between a short-term cycle (a), a long-term cycle involving

accumulation of CaCO₃ and organic carbon in marine sediments, their subduction, alteration and return of CO₂ to the atmosphere (b), and a medium-term cycle involving storage of carbon in organic materials in sedimentary rocks (c). Source:

Fig. 5.2 in Mackenzie and Mackenzie 1995.

Svensmark and Friis- Christensen (1997) have suggested a mechanism to explain the correlation. They point out that the cosmic radiation hitting the Earth de- creases with increasing sunspot activity since the solar wind de- flects the cosmic radiation.

Since increase in cosmic radiation increases the cloud

cover, the cloud cover tends to be largest near sunspot minimum (see figure 3).

They estimate the increase in cloud cover to be about 3 % from sunspot maximum to minimum and state that this corresponds to a radiative forcing of – (1 – 1.5) Wm^-2. Other studies indicate that the effect is much smaller. Thus Ringer & Shine (1997) found the radiative forcing for one per cent change in cloud cover to be -0.165 Wm^-2 using data from one satellite and in fact a positive value for other satellite data. In addition, Kuang et al. (1998) point out that the optical properties of clouds seems to more or less cancel out the effect of changes in cloud cover due to changes in cosmic radiation.

2.1.2 Variations in the Earth’s orbit

The evidence for amplifying feedback mechanisms in the climate system is a recurrent theme in almost all studies of climate change. Thus, the correlation between global glaciations over the last few million years and small changes in the incoming solar

radiation due to changes in the Earth’s orbit around the sun is another example. There is relatively clear evidence for long term climate cycles with the same periods changes as the Earth’s eccentricity (100 000 years), obliquity (41 000 years) and precession (19-23 000 years) in the records of past ice ages. The orbital changes (see figure 4) somehow seem to trigger global climate change. However, it is not well understood how these so called Milankovich cycles leads to the observed large scale synchronicity between the northern and southern hemisphere. Also the relation to the recorded changes in greenhouse gas concentration (CO₂ and CH₄) is less than fully understood.

Berger and co-workers have modelled variations during the past 200 thousand years (ka) of the Northern Hemisphere ice volume forced by insolation and reconstructed CO₂ levels (see figures 5 and 6). The agreement with other estimates of the ice volume variations is reasonably good. According to their model calculations, the variations in the Earth’s insolation, although small, induce feedbacks that are sufficient to generate large climatic changes. The change in albedo is being particularly important. This

supports the suggestion by Hays et al. (1976) that the orbital forcing acts as a pacemaker of the ice ages.

Figure 4. Variations in the Earth’s orbit. From: Andersen og Borns (1997) fig. 1-26.

They also ran the model for the coming 130 ka with a CO₂ scenario based on the recon- structed values for the past. An important result is that the ice volume will remain almost as today up to 50 ka AP (after present). This conclusion was a robust feature of the model studies.

Figure 5. Northern hemisphere insolation during the past 200 000 and the next 150 000 years. Source: Berger and Loutre (1998).

Figure 6. Estimates of northern hemisphere ice volumes during the last 200 000 years and the next 150 000 years. The forecasts are made based on three different

assumptions about future CO₂ content in the atmosphere. The lower curve is based on a CO₂ concentration equal to 200 ppmv, i.e. close to conditions under the last ice age. The middle curve assumes a concentration level of approximately 280 ppmv, corresponding to the level just before the industrial revolution. Finally, the upper curve assumes continued growth in the CO₂ concentration towards a level of 750 ppmv over the next 200 years, before the concentration declines to 300 ppmv over the following 450 years. Source: Berger and Loutre (1998).

2.1.3 The shape and positions of continents

Over geological time scales continents and oceans have changes position and shape.

For instance, 500 million years ago Norway was close to the South pole. During the journey to the present position all continents became assembled into a super-continent called Pangaea, which lasted until approximately 200 million years ago. This wandering of the continents has had great impact on the global climate, foremost through the increased likelihood for glaciation that appears when the polar regions are covered by continents, but also through changes in the heat transport carried out by ocean

currents. Thus, it is a current hypothesis that the onset of the last ‘ice house’ period with cycles between glacial and inter-glacial periods some 2 million years ago, was caused by the closing of the gap between the South and North American continents.

2.1.4 Albedo

The glaciation in itself creates a positive feedback mechanism for climate change since the high albedo of ice leads to high reflection of sunlight, thus cooling the Earth further.

To escape from this feedback loop, other factors affecting the climate must change.

Thus, it is now hypothesised that the Earth in a period several hundred million years ago actually froze over and became a snowball. Only intense volcanic activity with huge releases of CO₂ causing warming and particles darkening the ice cover, thus increasing the absorption of solar radiation, eventually lead to a melting of the ice and snow (Hoffman, 1998).

The cloud cover also affects the albedo or reflectivity of the Earth. The amount and position of clouds are in turn determined by the temperature, the humidity and the

concentration of aerosols of the atmosphere, as well as local topographic features like mountains. Thus, the creation of Rocky Mountain 100 million years ago, the Alps between 10 and 60 million years ago and the Himalayas 10 million years ago all affected the cloud cover of the Earth.

On a much shorter time scale, land use

change by humans, for instance in the form of deforestation, has affected the albedo of substantial parts of the Earth’s surface.

Table 1 shows some typical values of the albedo expressed as percentages. The average albedo of the Earth above the atmosphere is rougly 30%.

2.1.5 Atmospheric composition

Finally, the chemical composition of the atmosphere is a key factor in determining the global climate. Chemical constituents control the radiative balance of the Earth/ atmos- phere system due to interactions with both shortwave and longwave radiation, see figure 7. By absorption of terrestrial (longwave) radiation and re-emission at lower temperatures, the atmosphere is trapping radiative energy and thereby heating the surface-troposphere system. This mechanism keeps the surface about 33 °C warmer than it would otherwise be. The trapping of radiative energy, often referred to as "the

Table 1. Some examples on albedo values (in per cent) in the visible part of the spectrum.

Sand 18-28

Grass 16-20

Green plants 15-25

Forests 14-20

Dense forests 5-10

New snow 75-95

Old snow 40-60

Cities 14-18

Source: Houghton (1985).

greenhouse effect", is mainly due to the presence of water vapour, clouds and carbon dioxide (CO₂) in the atmosphere. Water vapour and clouds are the dominating factors in what can be called the natural greenhouse effect. Other gases such as methane (CH₄), nitrous oxide (N₂O) and ozone (O₃) also absorb and re-emit longwave radiation and contribute to the natural greenhouse effect. In addition to being radiatively active in the longwave region of the spectrum, ozone also absorbs solar (shortwave) radiation.

While CO₂ is the most important gas for the man-made enhancement of the greenhouse effect (responsible for approximately 60% of the warming effect since pre-industrial times), there are also significant contributions from methane (CH₄), nitrous oxide (N₂O), and halocarbons, see figure 8 and table 2.

4: Absorbed in clouds 16: Absorbed

by H2O, O3, dust

50 20

Net emission of infrared radiation from surface

6 2 4

Latent heat flux Sensible

heat flux Reflected

by surface Back-

scatter by air

Reflected by clouds

Net emission by H2O, CO2

Emission by clouds 14: Absorption

by H2O, CO2 Incoming

solar radiation

6 20 4

Reflected solar radiation

6 3 8 26

Outgoing infrared radiation 100

Figure 7. The Earth’s radiation balance. Incoming radiation (342 W/m²) set equal to 100.

Table 2. Summary of properties of important greenhouse gasses (IPCC, 1996, WMO, 1999).

CO₂ CH₄ CFC-11 N₂O

Concentration unit ppmv ppmv pptv ppbv

Pre-industrial concentration ~280 ~0.7 0 ~275

Concentration 1994 358 1,72 268 312

Recent increase per year 1.5

(0.4%) 0.010

(0.6%) 0

(0%) 0.8

(0.25%) Lifetime in the atmosphere

(year) (50-200)¹ 12³ 50 120

GWP (100 years horizon)² 1 24 500-2000 360

3. This has been defined as an adjustment time which takes into account the indirect effect of CH4 on its own lifetime.

N₂O C H₄

C O₂

Sulfate

B i o m a s s burning

Indirect e f f e c t s o f

a e r o s o l s F o s s i l

f u e l s o o t T r o p o s p h e r i c

o z o n e

S t r a t o s p h e r i c o z o n e H a l o -

c a r b o n s

-2 - 1 . 5 -1 - 0 . 5 0 0 . 5 1 1 . 5 2

Global-mean radiative forcing (Wm-2) H E A T I N G

C O O L I N G

Controlled by emissions of:

CO2 C H4

NOX

CO

N2O CFC-11

CFC-12 C C l4

H C F C - 2 2 C F C -

113

H a l o - c a r b o n s

(CFC H C F C H a l o n s )

NOX

C O V O C

C H4

S O2 Soot S-, N-

and C - g a s e s aerosols

SO2

aerosols

Extention: G l o b a l _Regional^Global/

Hemi- spheric/

R e g i o n a l

Regional Confidence

level: H i g h Low V e r y l o w

Figure 8. Change in the radiative balance at the top of the troposphere – called radiative forcing – since the beginning of the industrial area due to changes in concentration of various constituencies.

Source: Skodvin and Fuglestvedt (1997)

The depletion of the stratospheric ozone layer has caused a cooling effect, while the ozone increase in the underlying troposphere has probably given a warming that is of similar magnitude as the effect of methane. Sulphate particles formed from SO₂ and particles from biomass burning give a cooling effect due to scattering of solar radiation, while soot has a small warming effect through absorption of long wave radiation.

Finally, there may be a large cooling effect from changes in the distribution and

properties of clouds due to aerosols. This mechanism is, however, very poorly quantified at present.

We do not have the space here to go through the details, but note that due to the complexity of the chemical and other reactions taking place in the atmosphere, the uncertainties are quite large. We note for instance that water vapour (H₂O) is a potent greenhouse gas. The concentration of water in the atmosphere is, however, governed by the state of the atmosphere (temperature etc.) which in turn is influenced by the

presence of the gases mentioned above. Thus, the effect of water vapour on the climate is properly classified as a feedback effect in the climate system, rather than as a direct greenhouse gas effect. Furthermore, the translation of changes in radiative forcing to estimates of changes in temperature is also very difficult, and the uncertainty is of course amplified further when additional feedback mechanisms, for instance involving the oceans or the biosphere, are taken into account.

others are less so. However, the often non- linear coupling between them creates the largest obstacle to our understanding of the climate system. On the short time scale that is of importance to our human society, we note that the composition of the atmosphere together with the interactions between atmo- sphere, the oceans and the biosphere are of most relevance.

2.2 Remembrance of things past

Since its creation 4.6 billion years ago, the Earth has gone through enormous changes.

Continents have been born and reformed, the solar output has increased some 30 percent, and the oceans and the atmosphere have been created and changed. Given these

fundamental changes, it is a near miracle that life has evolved and managed to stay alive over much of Earth’s history.

Time scales of billions of years are difficult to

grasp. In order to make it more ‘digestible’, some memorable moments in the Earth’s history are listed in box 2¹.

The relatively slow start is noteworthy. It took more than a billion years before the first sign of life appeared in the ocean in the form of single cell bacteria, and almost three billion more years before the first animals were established, also in the ocean. One possible reason for this delay is that the Earth during the period from 750 to 550 million years ago may have frozen completely over (Hoffman et al., 1998). Only after melting of the snowball did land get occupied, first by plants and then gradually by animals mi- grating from the ocean. At this stage oxygen had become an important constituent of the atmosphere, which at earlier stages had been dominated by CO₂. Volcanic activity

and geological processes like weathering and erosion had kept the CO₂ concentration in balance at a high level during the first period. With the in- vasion of land, these processes were modified and a new bal- ance between the lithosphere, the oceans and the atmosphere was created. The balances were not perfect, however, and severe climate changes in the form of extensive glaciations took place in this early part of the history of the Earth.

1 See also Fortey, 1997.

history of the Earth Time (Million years ago)

4 600

The creation 3 300 First life

680 First animal 470 First fish 412 First plant

330 First tropical forest 215 First dinosaur 140 First bird

65 Dinosaurs die out 2.3 First homo

.100 First homo sapiens sapiens .040 Eurasia invaded by homo

saphiens

.015 Cave paintings in France and Spain

.010 The end of the last ice age .008 First civilization

.004 First cities

Adapted from C. Boyle (ed.)(1991): The Human Dawn, Time-Life Books B.V., Amsterdam.

Figure 9. Ice ages over the last 750 000 years

The first human like animal is some 2 million years old, i.e. very recent in a geological perspective. Homo sapiens are thought to have first appeared about 400,000 years ago, certainly in Africa and perhaps in parts of Asia as well. Anatomically modern humans appeared in Africa and possibly in Asia perhaps 100,000 years ago and eventually

arrived in Europe. Among these European peoples, the best known is the Cro-Magnons.

Their populations expanded rapidly throughout Europe, and their level of material cul- ture became increasingly more complex and sophisticated. The emergence of fully modern humans in other parts of the world is less understood, though it seems to have occurred 30,000-15,000 years ago and involved various migrations and the intermingling of different populations (see "human evolution" in Britannica Online²). Agriculture and stationary settlements seems to have appeared ca. 7 000 years ago, while the oldest remains of cities are some 4 000 years.

During this evolution towards civilisation, the Earth has gone in and out of so called ice houses, i.e. periods where more or less regular and extensive glaciation has take place.

Currently we are in such an ‘ice house’ which started some 1.6 million years ago and we have so far experienced 10 major and 40 minor periods of glacial and interglacial

conditions, see figure 9 for a record of ice ages over the last 750 000 years. The most recent ice age ended some 10 000 years ago and the present interglacial is expected to persist over an unusually long period. The ice sheets are not expected to appear for some 50 000 years.

100 200 300 400 500 600 700

0

50 000

100 000

150 000

200 000

250 000

300 000

350 000

400 000

Year before present CO2 concentration (ppmv)

Last 400 000 years: 180-300 ppmv Last 1000 years before 1750:

ca. 280 ppmv

Today: ca. 360 ppmv Year 2100, "business as usual":

ca. 700 ppmv

Figure 10. CO₂ concentrations over the last 400 000 years together with current and expected CO₂ concentration levels. Past measurements from Petit et al. (1999)

After the end of the last ice age the climate not only became generally warmer, but also in an important way more stable (see for instance Severinghaus et al., 1998). There are increasing evidence that not only during cold periods with extended glaciation, but also in the previous warmer inter-glacial periods, the climate was characterised by large variability on a short (decadal) time scale.

Only after the last ice age seems the climate to some extent to have quieted down. It is noteworthy that agriculture only emerged ca. 7 000 – 8 000 years ago, i.e. a couple of thousand years after the end of the last ice age and only after a quieter and more stable climatic period started. This event, or the establishments of cities some thousands of years later, can perhaps be said to represent the start of the civilisation as we know it.

Thus, our civilisation has only known our present calm and stable climate.

In this context it is instruc- tive to take a look at a graph (figure 10) showing CO₂ levels in the atmosphere during the last four ice ages.

Combined with current and future trends in CO₂ levels, the figure gives a vivid picture of the rate of change we are currently imposing on the atmospheric compo- sition. Already, the CO₂ concentration at approxi- mately 360 ppmv is far above anything we have experienced over the last 400 000 years. The near vertical increase in CO₂ concentration also gives an indication of the

unprecedented rate of change we now impose on the climate system.

Figure 11 shows the deve- lopment over the last 1000 years. While the CO₂ levels varied between approxi- mately 190 and 300 ppmv over the last 220 000 years before industrialisation, it has now reached ca. 360 ppmv during the last two centuries (i.e. an increase of ca. 30% from the pre-indus- trial level).

Figure 11. CO₂ concentration over the last 1000 years. Measurements since 1957, ice core estimates for earlier periods.

Source: IPCC (1996).

Figure 12. CH₄ concentration over the last 1000 years.

Source: IPCC (1996).

The concentration of methane (CH₄) has increased even more; 145% since pre- industrial time, see figure 12. The pre-industrial range of variation since 220 000 years before present was 300 to 700 ppbv, while the present level is 1720 ppbv. In addition to these changes, man's activities have introduced new gases to the atmosphere that sig- nificantly affect the fluxes of radiation. Of particular importance are the halocarbons containing fluorine, bromine or chlorine.

It has generally been assumed that there would be no period in the history of the earth where one could study the effects of a difference in emissions and uptake of

greenhouse gasses similar to what is found today. However, this seems to be the case about 55 million years ago. Recent studies (Dickens, 1999, and Norris and U. Rõhl, 1999) have shown some interesting changes during a short period of time where

concurrently with a rapid increase in temperature (5-7ºC at higher latitudes) there was a decrease in the isotope ratio ¹³C/¹²C. The studies conclude that there was an enormous emission of methane presumably from methanehydrates (these consist of methane gas and water and are stable at low temperatures, high pressure and high methane

concentrations). The total emissions are estimated to 1200 - 2000 GtC during less than 10 000 years; probably more than 600 GtC were emitted during less than 1000 years.

These emissions exceed both in amount and emission rate the current man-made emissions. From the beginning of the emission period until the earth had returned to pre-event conditions some 140 000 years had passed. The conditions 55 million years ago were of course different from those prevailing today. One must therefore be cautious in using these results to predict effects of toady’s emissions. However, the results are a new reminder that our actions today may affect the conditions on the earth for very many generations.

Taken together with the increasing acknowledgement of the potential natural instability of the climate system, also in warm inter-glacial stages, the picture presented above provides an important piece of motivation for the current concern about climate change and the work undertaken within the IPCC system.

2.3 The present

Returning to the opening statement from the Second assessment report of IPCC that

“The balance of evidence, … suggests a discernible human influence on global climate”, it is a fact that the statement drew some criticism. This was partly due to the difficulties encountered when interpreting the current signals on climate change such as global mean temperature, see figure 13. Although the wording of IPCC is very cautious, it remains debatable whether we in fact today observe ‘a discernible human influence’.

Recent studies such as those referred to in figure 14 and 15 from Mann et al. (1998, 1999), indicate, however, a steadily increasing role of greenhouse gases as an important explanatory factor behind the observed temperature increase.

What is not in doubt however, barring very large surprises, is that we in the future will see such influence on the global climate. Thus, the debate of the above statement is in a sense spurious and related only to a specific and very short period of time.

Annual mean temperature relative to 1961-1990

-0,6 -0,4 -0,2 0 0,2 0,4 0,6

1855 1860 1865 1870 1875 1880 1885 1890 1895 1900 1905 1910 1915 1920 1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

Degree Celsius

Global

Northern hem. 12 years running average Southern hem. 12 years running average Global 12 years running average

Source: P. D. Jones, D. E. Parker, T. J. Osborn and K. R. Briffa, Hadley Centre for Climate Prediction and Research, http://cdiac.esd.ornl.gov/trends/temp/jonescru/data.html

Figure 13. Average global mean temperature relative to the period 1961-1990. 12 years running average for the northern and southern hemisphere together with globale average.

Figure 14. Correlation between Northern Hemisphere temperature (NH), solar

Building on recent studies, Mann et al. reconstructed hemispheric temperature with proxy data networks for the past millennium. Though expanded uncertainties prevent decisive conclusions for the period prior to AD 1400, the results suggest that the latter 20th century is anomalous in the context of at least the past millennium. The 1990s was the warmest decade, and 1998 the warmest year, at moderately high levels of

confidence. The 20th century warming counters a millennial-scale cooling trend which is consistent with long-term astronomical forcing (figure 15).

Figure 15. Northern hemisphere temperature anomalies. From Mann et al. (1999) In addition to the analysis of development in global mean temperature, the statement from IPCC given above, relies also on so-called “fingerprint studies”. Changes in global mean temperature may have several causes, so in order to better understand the different factors behind climate change analyses taking seasonal, geographical and vertical patterns into account were performed. Comparing the results of model studies including the effects of greenhouse gases, sulphate aerosols (direct effect) and

stratospheric ozone depletion with the observed patterns of temperature changes gives an indication of man-made effects on the temperature pattern of the atmosphere in time and space. In a study by Tett et al. (1999) they present a quantification of the possible contributions throughout the century from the four components most likely to be responsible for the large-scale temperature changes, of which two vary naturally (solar irradiance and stratospheric volcanic aerosols) and two have changed decisively due to anthropogenic influence (greenhouse gases and sulphate aerosols). The patterns of time/space changes in near-surface temperature due to the separate forcing

components were simulated with a coupled atmosphere-ocean general circulation model, and a linear combination of these was fitted to observations. Thus the analysis is insensitive to errors in the simulated amplitude of these responses. Tett et al. find that solar forcing may have contributed to the temperature changes early in the century, but anthropogenic causes combined with natural variability would also present a possible explanation. The temperature change over the past 30-50 years is unlikely to be entirely

due to internal climate variability and has been attributed to changes in the

concentrations of greenhouse gases and sulphate aerosols due to human activity.

It is worth noting that the climate forcing from tropospheric ozone as well as the

indirect effect of aerosols have not been taken into account in the fingerprint studies so far. As can be seen from figure 8, these effects are probably significant components of the man-made interference with the energy balance of the Earth-Atmosphere system.

2.4 Future climate change

The future development of our global climate is determined by natural components and by man-made components. Concerning the possible future development of our global climate, IPCC has created a set of more or less likely emission scenarios based on various assumptions regarding population growth, economic development and

technological progress to quantify the importance of the man-made components³. The implications of these scenarios on some global climate indicators have then been worked out based on our best current knowledge on how the climate system may respond to radiative forcing, see figure 16.

Anthropogenic emissions (GtC) Concentration, (ppm)Global sea level change (cm)

(d)

(

Global change in temperature, deg.C

Figure 16. IPCC scenarios. The panels shows from top left to right, anthropogenic emission of CO₂, CO₂ concentration in the atmosphere, global change in

temperature and global sea level rise. Source: IPCC (1996)

Based on analyses like these, the IPCC estimates that we during the next century may face an increase in mean global temperature of between 1 and 3.5 °C and a sea level rise of between 15 and 95 cm above current levels. The lower figures are obtained by

employing a low emission scenario and assuming low climate sensitivity, i.e. a relatively low temperature increase per unit increase in radiative forcing. Similarly, the high numbers stems from combining a high emission scenario with relatively high climate sensitivity. It is worth noting, however, that assumption about future sulphate aerosol concentration (formed from SO₂ emissions) is crucial. Thus, assuming SO₂ emissions to stay constant at the 1990 level in the future, increases the maximum temperature estimate in 2100 to 4.5 °C above 1990 level.

During the last hundred years or so, the mean temperature has increased by

approximately 0.6 °C and the sea level has risen by between 10 and 25 cm. However, these global indicators do not really tell us very much about the regional and local effects of climate change. Thus, while

• precipitation is thought to increase under generally warmer conditions, and the distribution of precipitation is probably going to be more extreme in that dry places will get drier while wet places will get wetter,

• the warming will be more pronounced over land than over oceans,

• the warming will be strongest in the north at high latitudes,

• and more days with extreme heat and fewer days with extreme cold are expected, it is still too early to say with precision where these changes will take place. The task of determining the likely regional distribution of a possible global climate change is a main challenge for IPCC at the moment. We should, however, also recognise the possibility of surprises, i.e. the possibility of unexpected behaviour of the climate system as a response to human induced stresses.

2.5 The effects of the Kyoto Protocol

As a response to the threat of climate change, the international community negotiated and agreed on the so-called Kyoto Protocol in December 1997 (although it still needs to be ratified before entering into force). The protocol commits the industrialised countries (the so-called Annex B-countries) to reduce⁴ their total greenhouse gas emissions in the period 2008-2012 relative to their 1990 emissions by 5.2 percent. Calculations show that currents commitments under the Kyoto Protocol will have very small effects on future CO₂ levels, temperature change and sea level rise over the next century (Wigley, 1998).

In doing these calculations, assumptions are needed on future emission levels beyond the period regulated in the Kyoto Protocol (2008-2012). Wigley based his estimates on three different scenarios: a) No further reductions (designated NOMORE in figure 17), b) Annex-B emissions remain constant after 2010 (B=const.) and c) 1% annual decrease in CO₂ emissions (B=-1%). Furthermore, he explored the impacts of using three

different climate sensitivities in the calculations representing various strengths of the internal feedback mechanisms in the climate system. Conventionally, these are

measured as increase in global mean temperature in a scenario with CO₂ concentration equal to twice the value in pre-industrial times.

4 A few counties, Norway being among them, are allowed to increase their emissions somewhat.

In all cases the estimated reductions in global mean temperature due to the Kyoto Protocol are between 0.08 and 0.28°C for the year 2100, while the reductions in sea level rise are between 1.4 and 6.5 cm (see figures 17a and b).

Figure 17a. Upper part of the figure: Global mean sea level changes for the baseline (IS92a) and the extended Protocol emissions cases using a climate sensitivity of 2.5°C warming for 2xCO₂. Lower part of the figure: Reductions in global sea level rise in the scenario with constant emissions in Annex B nations after 2010 for climate sensitivites of 1.5, 2.5 and 4.5°C (Wigley, 1998).

Figure 17b: As figure 17a, but for global mean temperature changes (Wigley, 1998).

2.6 Unresolved scientific questions

As emphasised earlier, the climate system is composed of several sub-systems interlinked in a non-linear fashion. Such systems are known to be able to behave in erratic and surprising ways. Any long-term forecast of climate change is therefore conditioned on the assumption that the system is only perturbed within boundaries where it behaves smoothly in some sense. Imprecise knowledge about where these boundaries are located gives additional uncertainty to the forecasts. However, even within this constraint, several important sub-systems and mechanisms remain less than well understood.

An intriguing part of the climate system in this respect is the clouds. Low altitude clouds are known to be mainly cooling, due to the increased albedo they entail and the

relatively high temperature of the cloud tops. This high temperature ensures relatively intense outwards thermal radiation from the clouds. Oppositely, clouds at high altitude with cool cloud tops contribute mainly to global warming since their upward thermal radiation is much less. How increased greenhouse gas emissions, and also emissions of other pollutants leading to aerosol formation, affect the formation of clouds at different altitudes and their optical properties, is not well understood. Varying assumptions related to these mechanisms are often the main explanation why different climate models predict different increases in the mean global temperature from the same increase in greenhouse gas emissions.

Another indication on our incomplete knowledge of the climate system is the discrepancy between trends in observed surface temperatures and satellite measure- ments of the temperature of the lower troposphere (close to ground level). While unambiguous global warming over the last few decades is observed at surface stations, the satellite observations of the lower troposphere are more ambiguous (Christy, 1998).

This may be a sign that the thermal coupling between the surface and the lower part of the atmosphere is more complex than assumed in most of the currently operating climate models. It may also indicate that the technique behind monitoring temperatures from satellites still is at an early stage. Over the last years, several corrections and improvements of the satellite series have been done, leading to a better agreement with the surface measurements. In addition to the temperature records mentioned above, surface temperature trends have been derived from temperature measurements in boreholes. The studies indicate that the Earth’s mean surface temperature has increased by about 0.5 °C in the 20th century and that this century has been the warmest of the past five centuries (Pollack et al., 1998).

Other substantial questions are related to the coupling of the atmosphere and the oceans, and the many natural cycles observed in the oceans (El Niño, North Atlantic Oscillation, etc.). These have a great impact on the regional climate. However, the coupling to the development of global climate change and vice versa is still unclear.

Similar uncertainty exists with regard to the coupling to the biosphere and its responses to climate change.

Finally, the role of changes in solar activity is still not clear. As mentioned in section 2.1.1 there are some indications that the influence solar activity on our climate is incompletely understood at present.

Perhaps a good illustration of our incomplete understanding can be found in the temperature increase observed between 1910 and 1940 in the historical record, see figure 13. Certainly a better understanding of climate change over this period will in- crease our ability to predict future climate change with greater precision.

Regardless of our incomplete understanding of the climate system, some conclusions can be stated with a high degree of confidence (see also Box 3).

- The climate system, as an important part of the natural environment, has a major impact on our civilisation and way of life. Without a reasonable stable climate our way of life is going to be difficult, perhaps even impossible in the present form.

- We are currently interfering significantly with the climate system. Although this can not be proven beyond doubt at the present, the interference is steadily increasing over time. A possible transition to a more unstable climatic regime can not be

excluded as a consequence of this disturbance. The potential adverse effects of such a change in climatic regime, make it reasonable to adopt a defensive attitude.

- Adoption of a defensive attitude is dependent on political and public acceptability.

Thus, a main priority at present is to bring forward scientific information about what is known and what is currently unknown to the public at large and the political decision makers in particular.

Box 3. Knowledge status

This we know This is probable This is uncertain

• We do have a natural greenhouse effect caused by the presence of clouds and

greenhouse gases in the atmosphere. The most important greenhouse gases are: H₂O, CO₂, CH₄, N₂O and O₃.

• Since pre-industrial times the

concentrations of CO₂, CH₄ and N₂O have increased by 30%, 145%

and 15%, respectively.

• Over the last 100 years the mean global tempe- rature has increased by 0.3-0-6 °C with a series of record breaking years in the late 1980s and the 1990s.

• Anthropogenic

emission of greenhouse gases has probably contributed significantly to the observed changes in climate.

• With current develop- ment in emissions we can expect by year 2100:

- Global increase in tem- perature of 1-3.5 °C.

- An increase in sea level of some 15-95 cm, and additional increased af- ter 2100.

- Loss of agricultural land.

- Changes in

precipitation patterns.

- Changes in eco- systems.

- Increased frequency of certain illnesses, e.g.

• Large and abrupt

changes in climate have happened previously in the Earth’s history. We know little of the causes of these changes.

• The are large

uncertainty regarding the regional impacts of a change in the global climate.

• There is a lack of know- ledge about

- The feedback mecha- nisms, in particular re- lated to the humidity of the atmosphere and cloud formation.

- The cooling effects of aerosols and sulphur emissions.

- Links to the sunspot cycle.

over the same period. other extreme climatic events.

- Changes in strength and pattern of the oceanic currents.

3 IPCC: The background, organisation and procedures

3.1 Background: The history of the establishment of IPCC

Scientific recognition of the potential of human activity to modify climate dates back at least to the early nineteenth century. Thus, in 1827, Baron Jean-Baptiste Fourier

suggested that human activity can modify surface climate, and he was perhaps the first to suggest the now well known greenhouse effect of the atmosphere (Fourier, 1827, Ramanathan, 1988). The greenhouse theory of climate change was, however, only taken up in earnest later in the last century when in 1896 the Swedish scientist Svante Arrhenius published his first estimate of a man-made global temperature change caused by industrial emissions (Arrhenius, 1896, Rodhe et. al., 1997). His main insight was that burning of fossil fuels and the release of CO₂ could affect the escape of heat from the Earth.

The next milestone can perhaps be said to relate to research carried out by Roger Revelle and Hans Suess at the Scripps Institute of Oceanography.

Their research indicated that the oceans only seem to absorb about half on the man made CO₂ emissions to the at-

mosphere. This research led in turn to the establishment of a monitoring network under the guidance of Charles Keeling from the same institute. This monitoring firmly established that the CO₂ concentration in our atmosphere is increasing and is now far above the level believed to have existed in pre- industrial times (280 ppmv), see figure 18.

Scientific interest in man’s potential impact on global climate was stirred by the research and monitoring initiated in the 1950’s, and this interest was further mobilised through conferences, loose research networks and assessments especially from the 1970’s onwards (Agrawala, 1998a, b).

The starting point for the recent international efforts to better understand climate variations and the possible problem of a human-induced climate change is generally

Figure 18. Measurements of CO₂ concentrations at Mouna Loa, Hawaii

with a report from the University of East Anglia highlighting that the 1980s contained several of the warmest years in the historical record, created widespread concern about climate change as a man-made global environmental problem.

In 1979 the World Climate Conference was held in Geneva, and the World Climate Pro- gramme (WCP) was launched. The creation of the WCP set forth a series of workshops held in Villach, Austria, in 1980, 1983 and 1985, and organised under the auspices of the World Meteorological Organization (WMO), the United Nations Environment

Programme (UNEP) and the International Council of Scientific Unions (ICSU) (Agrawala, 1998). At the 1985 Villach meeting an international group of scientists reached a consensus that, as a result of the increasing concentrations of greenhouse gases in the atmosphere, a rise in the global mean temperature “greater than any in man’s history” could occur in the first half of the next century. This group of experts also stated that “…the understanding of the greenhouse question is sufficiently

developed that scientists and policy-makers should begin active collaboration to explore the effectiveness of alternative policies and adjustments” (WMO, 1985).

In combination with a set of other factors, especially anomalous weather conditions in Europe and America, the 1985 Villach meeting was instrumental in bringing the climate issue onto the international political agenda. In 1986 the Advisory Group on

Greenhouse Gases (AGGG) was set up under the joint sponsorship of WMO, UNEP and ICSU. Each of these bodies nominated two experts, and the panel consisted of six members: Gordon Goodman, Bert Bolin, Ken Hare, G. Golitsyn, Sukiyoro Manabe and M. Kassas (Agrawala, 1998).

During the latter half of the 1980’s the climate issue increasingly gained saliency among the public, scientists and policy-makers, not least through the work of the so-called Brundtland commission (WCED, 1987). At the Toronto Conference of the Atmosphere, where more than 300 scientists and policy-makers from 48 countries, UN organisations, IGOs and NGOs participated, an explicit policy recommendation calling upon national governments to reduce CO₂ emissions by 20% from 1988 levels by 2005 was agreed upon.

Meanwhile, the WMO and UNEP in close co-operation with various US agencies agreed that an intergovernmental mechanism was needed to undertake further

internationally co-ordinated scientific assessments of climate change, and invitations to governments to the first session of the Intergovernmental Panel on Climate Change (IPCC) were sent out early 1988. The first plenary session of the IPCC took place in November 1988. The AGGG set up in 1986 was gradually replaced by the IPCC and has not met since 1990.

3.2 The function and products of the IPCC

The main function of the IPCC is to provide assessment reports of state-of-the-art knowledge on climate change. The objective of the IPCC, as formulated by the governing bodies of WMO and UNEP, is twofold:

i) To assess the scientific information related to the various components of the climate change issue and the information needed to evaluate the environmental and socio-economic consequences of climate change, and

ii) To formulate “realistic response strategies for the management of the climate change issue” (Report of the first session of the IPCC).

Figure 19. IPCC organisation

In 1988, three Working Groups (WGs) were set up to attain this objective (figure 19):

• Working Group I (WGI) was assigned the task of assessing available scientific in- formation on climate change,

• Working Group II (WGII) was assigned the task of assessing environmental and socio-economic impacts of climate change, and

• Working Group III (WGIII) was assigned the task of formulating response strategies.

In 1992, the IPCC structure was slightly changed: Working Groups II and III were merged in Working Group II, while a new Working Group III was set up to deal with socio-economic and other cross-cutting issues related to climate change.

IPCC has as one of its main tasks to assess “scientific information”. All IPCC WGs conduct assessments on the basis of published literature within relevant fields and disciplines. Thus, IPCC do not, contrary to a common misunderstanding, carry out scientific research. Furthermore, the term “scientific information” is generally taken to mean that only published or other peer reviewed scientific material is taken into account.

Authors * Contributors * Experts

Working group II

J. McCarthy USA

O. Canziani Argentina

Techical Support Unit Techical

Support Unit

UNEP/WMO

IPCC Chair R.T. Watson

USA

IPCC Bureau

IPCC Secretariat N. Sundaraman

Geneva

Working group III

B. Metz The Netherlands

O. Davidson Sierra Leone

Techical Support Unit

Working group I

J. Houghton UK

Ding Yihui China

• Working Group I will assess the scientific aspects of the climate system and climate change (as before);

• Working Group II will assess the scientific, technical, environmental, economic and social aspects of the vulnerability (sensitivity and adaptability) to climate change of, and the negative and positive consequences (impacts) for, ecological systems, socio- economic sectors and human health, with an emphasis on regional sectoral and cross-sectoral issues;

• Working Group III will assess the scientific, technical, environmental, economic and social aspects of the mitigation of climate change, and through a task group (multi- disciplinary team), will assess the methodological aspects of cross-cutting issues (e.g., equity, discount rates and decision making frameworks).

Box 4. IPCC reports:

• Assessment Reports: The full scientific assessment with status as “Reports accepted by WGs”. Accepted by WG plenary, but not subject to discussion.

• Executive summaries and Summaries for Policy-makers: Summaries of the full scientific assessment with status as “Reports approved by WGs and accepted by the Panel”. Subject to line-by-line approval by WG plenary. Accepted by full panel plenary, and not subject to discussion at this decision-making level.

• Synthesis Reports: Synthesis of the reports of all WGs, developed by the WG leadership in co-operation with lead authors and specially invited experts with status as “Reports approved by the Panel”. Subject to line-by-line approval by full panel plenary.

• Special Reports: Assessments on special issues. Subject to the review, acceptance and approval procedures of the assessment reports in general.

• Technical Papers (since 1995): Reports on specific issues, based on existing as- sessment reports, not submitted to the acceptance and approval procedures of the assessment reports.

The main products of IPCC are the assessment reports. However, also other types of reports are produced, see box 4. The First IPCC Assessment Report was presented to the Second World Climate Conference in 1990, where it was accepted as an adequate basis upon which to start climate negotiations. The first step was the Framework Convention on Climate Change (FCCC) agreed upon in Rio de Janeiro in 1992. In December 1995, the IPCC Plenary accepted the Second IPCC Assessment Report in time for the negotiation of the Kyoto-Protocol finalised in December 1997. Work on a Third Assessment Report (TAR) is underway (current work plans suggest finalisation in 2001).

3.3 The Assessment Process

The IPCC is organised in three decision-making levels that serve different functions in the assessment process: the “scientific core”, the WG plenaries, and the full panel (IPCC) plenary at the top of the institution, see figure 20.

At WG and panel plenaries, all UN members and members of the IPCC’s two

sponsoring organisations, the WMO and UNEP, can participate. Participation at these levels is, therefore, in principle open. Governmental authorities nominate all members of national delegations.

At the start of an assessment process, the leadership of each WG develops a work-plan for the assessment, which is subsequently approved by the plenary of the WG and accepted by the full panel plenary. Governments nominate teams of lead- and

contributing authors. The bureau (chair and vice-chairs) of each WG select lead authors from the nomination lists provided by governments. Contributing authors may also be specially invited; however, with due consideration of the geographic balance of the groups, particularly with regard to ensuring participation by scientists from developing countries. Lead authors participate in their personal capacities.

The assessment reports are developed in the scientific core of the IPCC, in a series of meetings in task forces and sub-groups established for particular issues, workshops and conferences, and most importantly, in regular lead- and contributing author meetings.

The summaries to the assessments – the summary for policy-makers and the executive summary – are also developed at this level. Scientists active in research dominate participation in the scientific core.

When a draft report has been developed, it is submitted to an extensive, two-phased review

procedure, including both expert and government review. According to the rules of procedure of the IPCC, lead authors, WG chairs, sub-group chairs and vice-chairs are responsible for incorporating comments from the review “as

appropriate”. In this regard, lead authors, chairs and vice-chairs are encouraged to arrange wider meetings with principal contributors and reviewers to discuss particular aspects or areas of major differences, as

deemed necessary and if time and funding permits. It is also emphasised in the rules of procedure that the assessment reports “describe different (possibly

controversial) scientific or technical views on a subject, particularly if they are relevant

IPCC

Full Panel Plenary

Working Group Plenaries

The Scientific Core of the WGs

Political Dominance

Science and Politics Balanced

Scientific Dominance

PURE POLITICS

The Climate Change Political Community

The UNFCCC

The Climate Change Scientific Community Scientific Community

PURE SCIENCE

Figure 20. The different levels of the IPCC process